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Employee Benefits (Components of Net Periodic Benefit Cost) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Loss) (on a pre-tax basis):      
Net (gain) loss $ 1,591 $ 4,106 $ (1,478)
Prior service cost [1] (15) (15) (17)
Recognized in other comprehensive (loss) income (154) (3,055) 2,948
Qualified Pension Plan [Member]      
Net Periodic Benefit Cost:      
Service cost 2,149 2,148 2,459
Interest cost 2,673 2,576 2,928
Expected return on plan assets (4,942) (4,633) (4,515)
Amortization of prior service credit (23) (23) (23)
Recognized net actuarial loss 1,114 828 1,249
Net periodic benefit cost 971 896 2,098
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Loss) (on a pre-tax basis):      
Net (gain) loss (1,798) 2,904 (2,816)
Prior service cost 23 23 23
Recognized in other comprehensive (loss) income (1,775) 2,927 (2,793)
Total recognized in net periodic benefit cost and other comprehensive income (loss) (804) 3,823 (695)
Non-Qualified Retirement Plans [Member]      
Net Periodic Benefit Cost:      
Service cost 129 122 78
Interest cost 428 432 490
Expected return on plan assets 0 0 0
Amortization of prior service credit (1) (1) (1)
Recognized net actuarial loss 347 247 245
Net periodic benefit cost 903 800 812
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Loss) (on a pre-tax basis):      
Net (gain) loss 1,928 127 (156)
Prior service cost 1 1 1
Recognized in other comprehensive (loss) income 1,929 128 (155)
Total recognized in net periodic benefit cost and other comprehensive income (loss) $ 2,832 $ 928 $ 657
[1] The pre-tax amount is included in salaries and employee benefits expense in the Consolidated Statements of Income.